PubSub and Client Server are both based on the OPC UA Information Model. PubSub therefore can easily be integrated into OPC UA Servers and OPC UA Clients. Quite typically, a Publisher will be an OPC UA Server (the owner of information) and a Subscriber is often an OPC UA Client. Above all, the PubSub Information Model for configuration (see 6.2.2) promotes the configuration of Publishers and Subscribers using the OPC UA Client Server model.
Nevertheless, the PubSub communication does not require such a role dependency. I.e., OPC UA Clients can be Publishers and OPC UA Servers can be Subscribers. In fact, there is no necessity for Publishers or Subscribers to be either an OPC UA Server or an OPC UA Client to participate in PubSub communications.